Optical waveguides – With disengagable mechanical connector – Optical fiber to a nonfiber optical device connector
Reexamination Certificate
1999-12-20
2001-11-13
Ngo, Hung N. (Department: 2874)
Optical waveguides
With disengagable mechanical connector
Optical fiber to a nonfiber optical device connector
Reexamination Certificate
active
06315465
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical module comprising an optical device which is a light-receiving device or light-emitting device.
2. Related Background Art
Optical modules such as receiving modules and transmitting modules are used in optical communication systems such as optical data links, optical LAN, and the like. A receiving module is equipped with a light-receiving device, for converting an optical signal transmitted through an optical fiber into an electric signal and outputting thus obtained electric signal; and a transmitting module is equipped with a light-emitting device, for converting an electric signal into an optical signal and sending out thus obtained optical signal to an optical fiber. Conventional examples of such optical modules are disclosed in Japanese Utility Model Application Laid-Open No. HEI 2-126107 and Japanese Patent Application Laid-Open No. HEI 2-278212.
An optical module comprises an optical device which is a light-receiving device or light-emitting device, and an electronic circuit connected to the optical device. In a conventional optical module, electronic components and wiring patterns constituting the electronic circuit are formed on a substrate or an island portion (substrate portion) of a lead frame functioning as a substrate, whereas the wiring pattern on the substrate and lead pins are connected to each other by wire bonding. Also, the electronic circuit and substrate, and individual components such as a metal package including an optical device are integrally molded with an insulating molding resin, so as to construct the optical module.
In the above-mentioned optical module, however, since the electronic circuit on the substrate and the lead frame are connected by wire bonding, it is necessary to provide the periphery of the substrate for wire bonding. Therefore, the size of the wiring substrate is restricted because of the necessity of the periphery region.
On the other hand, there has been known an optical module in which a lead frame is provided with an optical device mounting section and an electronic circuit mounting section which contain an optical device and an electronic circuit thereon respectively, and separately molded with a transparent resin (Proceedings of 48th Electronic Components & Technology Conference (1998), p. 1199). According to such a structure, the optical device and the electronic circuit are collectively mounted on the same lead frame.
In the structure mentioned above, the lead frame is formed by such a method as etching a sheet material or punching it with a pressing machine, whereby the wiring pattern of the electronic circuit is formed as well. Since the lead frame is formed from a single sheet material as such, however, cross wiring by which two or more wires are crossed cannot be made, whereby the degree of freedom in the electronic circuit configuration would be limited. For this matter, cross wiring may be realized by carrying out wire bonding in a part where crossing is necessary. In this case, however, impedance mismatching may occur, thus deteriorating circuit characteristics of the electronic circuit.
Also, in the case where a wiring pattern is formed by a lead frame, outer lead pins are formed simultaneously, whereby it is hard to form a wiring pattern with a wiring pitch not greater than the sheet thickness of the lead frame (on the order of 0.2 to 0.25 mm) due to conditions such as the strength of the lead frame and the easiness in its handling. Consequently, this technique cannot be utilized for forming complicated electronic circuits.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide an optical module having a configuration which can increase the flexibility of its electronic circuit.
For achieving such an object, the optical module in accordance with the present invention is an optical module comprising an optical device for converting one of an optical signal having a predetermined wavelength and an electric signal corresponding thereto into the other signal, an electronic circuit for processing the electric signal, and a lead frame having a lead pin connected to the electronic circuit; wherein a wiring substrate formed with at least a part of the electronic circuit is secured to one surface of the lead frame; wherein an electrode provided in the wiring substrate and a connecting portion provided in the lead frame are directly connected to each other, so that the electronic circuit on the wiring substrate and the lead frame are electrically connected to each other, the wiring substrate and the connecting portion of the lead frame being encapsulated with a resin so as to form an electronic circuit encapsulating portion; and wherein, in the electronic circuit encapsulating portion, a difference between a thickness of the encapsulating resin on the one surface of the lead frame from a surface of the wiring substrate opposite from the lead frame and a thickness of the encapsulating resin on the other surface of the lead frame is smaller than a thickness of the wiring substrate.
In this optical module, not only the electronic components and the wiring pattern on the lead frame but also the wiring substrate mounted on the lead frame is used for constructing the electronic circuit. As a consequence, the degree of freedom in the electronic circuit configuration can be enhanced such that cross wiring is formed within the substrate, for example. Also, the wiring pitch can be made sufficiently small, so as to meet increase in the complexity of electronic circuits.
Also, instead of using wire bonding for electric connection between the electronic circuit on the wiring substrate and the lead frame, the connecting portion is provided in the lead frame so as to face the electrode in the wiring substrate. The electrode and the connecting portion are secured and electrically connected to each other by use of an electrically conductive connecting member or the like. Here, it is unnecessary to provide the periphery of the wiring substrate for wire bonding, and no wiring pad for the lead frame is required, whereby the wiring substrate can be made larger, so as to increase the area of the electronic circuit.
In such a configuration, since the wiring substrate and the resin for encapsulating the wiring substrate so as to form the electronic circuit encapsulating portion have their respective coefficients of thermal expansion different from each other, the encapsulated resin may break after the encapsulation. In particular, in the case where the encapsulating resin of the electronic circuit encapsulating portion is a transparent resin, since the amount of fillers added to the transparent resin is small, a transparent epoxy resin, for example, has a thermal expansion coefficient on the order of 6.2 to 17.2×10
−5
/° C., which is at least 2 times larger than that of normal epoxy resins. On the other hand, a ceramic substrate such as alumina (Al
2
O
3
), which is often used as a wiring substrate, has a thermal expansion coefficient of about 5.5×10
−6
/° C., for example, thus yielding a difference of 10 times or greater therebetween. Though the difference is slightly smaller as compared with the transparent epoxy resin, similar problems exist in the normal epoxy resins as well.
In particular, in the case where the wiring substrate is mounted on the lead frame, if the thickness of encapsulating resins on the upper and lower sides of the lead frame are same, then the encapsulating resin on the wiring substrate will become thinner than the thickness of the encapsulating resin on the other side by the thickness of the wiring substrate. Here, due to the structural asymmetry of the encapsulating resins, stresses generated in the respective resin portions would be out of balance, whereby distortions and cracks in the encapsulating resin may occur at the portions where excessive stresses concentrate.
For this matter, in the optical module in accordance with the present invention
Mikamura Yasuki
Mizue Toshio
Tonai Ichiro
McDermott & Will & Emery
Ngo Hung N.
Sumitomo Electric Industries Ltd.
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